1. Field of the Invention
The invention relates to a method for detecting and selecting foreign parts in cigarettes, in which the tobacco is exposed to microwave radiation in the area of the cigarette maker (i.e. a cigarette making machine) and both a signal SD relating to the tobacco density and a signal SF relating to the tobacco moisture are generated.
2. Discussion of Prior Art
From German DE 197 05 260 A1 both a method and an apparatus are known for detecting at least one characteristic of a substance like the moisture mass and/or dry mass of tobacco by evaluating the unbalanced of microwave resonator which is caused by the presence of the substance, wherein microwaves with two differing frequencies are applied to the resonator and the resonator frequency shift and the dampening are analysed in comparison to the resonance curve which is free of the substance.
The EP 0 889 321 A1 discloses a microwave resonator for measuring the density profile and/or the moisture profile in longitudinal direction of a sample with high precision and local resolution, wherein the resonator is filled with a dielectric and with the cavity having a substantially smaller thickness than the lateral dimensions measured under a dimension orthogonal thereto.
In the tobacco industry it is necessary for the purpose of maintaining product quality to detect and remove foreign parts contained in the raw tobacco. The term foreign part is understood to mean any non-tobacco material.
The U.S. Pat. No. 4,707,652 discloses a method for the detection of foreign parts in cigarette tobacco by the application of electromagnetic stray radiation, which is analysed.
The U.S. Pat. No. 5,476,108 also relates to the detection of foreign parts in cigarette tobacco, the tobacco being examined with near infrared radiation if a foreign part is detected, a signal is generated for an ejector which includes a pressurised air nozzle, said nozzle thereupon directing an air jet against the cigarette containing said foreign part to remove it from the work flow.
Lastly, German DE 297 19 600 U1 discloses an apparatus for detecting inhomogenities and foreign parts in a material layer, wherein the material layer is treated with particle beams or electromagnetic radiation and the transmitted or reflected intensity in analysed.
These methods for the detection and removal of foreign parts in the cigarette maker are only usable to a limited extent and consequently, based on all foreign parts which might occur, have only an inadequate efficiency.
Apart from purely metal detecting devices or air and screen classifying methods, the prior art more particularly uses optical image evaluation methods in the tobacco processing field.
These methods suffer from the following disadvantages:                The effect of such installations is based on the optical detection of foreign parts. Therefore the tobacco mass stream must be thinned into a monolayer, which can only be brought about through high costs on equipment and space.        The detection rates are material-specific and in particular highly dependent on the colour and size of the foreign parts. Small foreign parts having a similar colour to tobacco are virtually undetectable.        Apart from the actual foreign parts methodically always good tobacco material is also discharged. This increases undesired tobacco losses and leads to additional costs.        In order that the detection and removal of foreign parts in conventional mass streams have any prospect of success, the foreign part detection installations for cigarettes are preferably located upstream of the tobacco cutting stage. However, not only is the raw tobacco contaminated with foreign parts, but also during the tobacco processing operation there is a potential risk of foreign parts entering the tobacco. Thus, despite the installed foreign part seeking installations, there is a risk that foreign parts can still enter the cigarette maker.        Admittedly on the cigarette maker there is a functional unit for the separation of coarse parts, such as e.g. small and undesired stem fragments or so-called winnower stems, but this unit is unable to remove foreign parts with an adequate efficiency, so that there is a risk of foreign parts ultimately entering the cigarette.        